Conventionally, as a backlight device of a liquid crystal display, devices equipped with a cold cathode fluorescent lamp as a light source have been widely used. There are some types of such display devices which are called as an edge light type and a direct type. The edge light type device includes a thin tubes of the cold cathode fluorescent lamps arranged at a side edges of a light guide plate, and the light which has been input from the side edge repeatedly reflects in the light guide plate, and then emitted from a main surface of the light guide plate.
On the other hand, the direct type backlight device includes a combination of a plurality of the cold cathode fluorescent lamps arranged in parallel, a reflection plate provided behind the cold cathode fluorescent lamp, and a light diffusion plate serving as a light emitting surface. In contrast to the edge light type, the number of the cold cathode fluorescent lamps can be increased, and therefore, brightness on the light emitting surface can be easily increased.
However, the direct-type backlight device has a problem of poor brightness uniformity on the light emitting surface. Particularly, periodical unevenness in brightness caused by the high brightness at portions right above the cold cathode fluorescent lamps is a major problem. That is, if the brightness uniformity on the light emitting surface is poor, unevenness in display occurs on a display screen of a liquid crystal display.
In the direct-type backlight device, it is possible to improve the brightness uniformity by decreasing intervals between the cold cathode fluorescent lamps. However, such a solution results in an increased number of the cold cathode fluorescent lamps, leading to complicated manufacturing process and increased power consumption for lighting. Alternatively, it is possible to improve the brightness uniformity by increasing the distance between the cold cathode fluorescent lamps and the light diffusion plate. In that case, however, the backlight becomes thick, and a liquid crystal display cannot be made thin.
Conventionally, various measures to improve the brightness uniformity have been taken. For example, there have been proposed a method in which a light amount correction pattern consisting of a strip or a dot pattern is printed on the light diffusion plate to decrease light beam emitted right above the cold cathode fluorescent lamps (Patent Document 1, FIG. 6), and a method in which a wavy reflection plate is used to focus reflection light from the reflection plate in a region corresponding to midpoint between the cold cathode fluorescent lamps (Patent Document 2).
However, if the light amount correction pattern is printed as means for improving the brightness uniformity, a part of the light beam is blocked. Therefore, utilization ratio of the light beam emitted from the cold cathode fluorescent lamps decreases, leading to insufficient brightness. The application of the wavy reflection plate complicates the configuration of the device.
For producing the light diffusion plate used in the direct-type backlight device, a material containing a transparent resin and a light diffusing agent dispersed therein is often used. However, if concentration of the light diffusing agent is increased to improve the brightness uniformity, brightness is decreased. In order to solve this problem, there has been proposed a light diffusion plate having a pattern such as a prism form on the surface thereof for exerting diffusing effect by the surface pattern without decreasing the brightness (Patent Documents 3, 4, and 5). However, such a prismatic pattern on the surface of the light diffusion plate alone can not achieve sufficient improvement in the brightness uniformity.
List of the Prior Art Documents:    Patent Document 1: Japanese Patent Application Laid-open No. H6-273760, FIG. 6    Patent Document 2: Japanese Patent Application Laid-open No. 2001-174813 A    Patent Document 3: Japanese Patent Application Laid-open No. H5-333333 A    Patent Document 4: Japanese Patent Application Laid-open No. H8-297202 APatent Document 5: Japanese Patent Application Laid-open No. 2000-182418 A